U.S. Pat. No. 2,950,799 describes filtering ultraviolet light, placing a polarising screen and band-pass filter in front of the photo-electric cell detecting the fluorescent radiation. The current measured is amplified and applied to a galvanometer provided with a contact. However, despite the polarisation and filtering of the fluorescent radiation, daylight or light coming from an artificial source using gas may influence the photo-electric detection cell by reflection and diffusion of the light in the violet range of the spectrum. Furthermore, detection errors may occur on account of a variation in the intensity of the ultraviolet source, for example when the lamp is changed, the intensity of spectral rays for the same supply current varying from one lamp to another and on account of the considerable ageing to which known spectral lamps are subject.